Field of the Invention
The invention relates to a bulk crystal of semiconductor material used to produce semiconductor wafers for various devices including optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs), and electronic devices such as transistors. More specifically, the invention provides a bulk crystal of group III nitride such as gallium nitride. The invention also provides a method of selecting seed crystals for growth of group III nitride bulk crystals.
Description of the Existing Technology
This document refers to several publications and patents as indicated with numbers within brackets, e.g., [x]. Following is a list of these publications and patents:    [1] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 6,656,615.    [2] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 7,132,730.    [3] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 7,160,388.    [4] K. Fujito, T. Hashimoto, S. Nakamura, International Patent Application No. PCT/US2005/024239, WO07008198.    [5] T. Hashimoto, M. Saito, S. Nakamura, International Patent Application No. PCT/US2007/008743, WO07117689. See also US20070234946, U.S. application Ser. No. 11/784,339 filed Apr. 6, 2007.    [6] D' Evelyn, U.S. Pat. No. 7,078,731.    [7] Wang et al., Journal of Crystal Growth volume 318 (2011) p 1030.
Each of the references listed in this document is incorporated by reference in its entirety as if put forth in full herein, and particularly with respect to their description of methods of making and using group III nitride substrates.
Gallium nitride (GaN) and its related group III nitride alloys are the key material for various optoelectronic and electronic devices such as LEDs, LDs, microwave power transistors, and solar-blind photo detectors. Currently LEDs are widely used in displays, indicators, general illuminations, and LDs are used in data storage disk drives. However, the majority of these devices are grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide because GaN substrates are extremely expensive compared to these heteroepitaxial substrates. The heteroepitaxial growth of group III nitride causes highly defected or even cracked films, which hinder the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
To solve fundamental problems caused by heteroepitaxy, it is indispensable to utilize crystalline group III nitride wafers sliced from bulk group III nitride crystal ingots. For the majority of devices, crystalline GaN wafers are favorable because it is relatively easy to control the conductivity of the wafer and GaN wafer will provide the smallest lattice/thermal mismatch with device layers. However, due to the high melting point and high nitrogen vapor pressure at elevated temperature, it has been difficult to grow GaN crystal ingots. Currently, the majority of commercially available GaN substrates are produced by a method called hydride vapor phase epitaxy (HVPE). HVPE is one of vapor phase methods, which has difficulty in reducing dislocation density less than 105 cm−2.
To obtain high-quality GaN substrates for which dislocation density is less than 105 cm−2, various growth methods such as ammonothermal growth, flux growth, high-temperature solution growth have been developed. Ammonothermal method grows group III nitride crystals in supercritical ammonia [1-6]. The flux method and the high-temperature solution growth use a melt of group III metal.
Recently, high-quality GaN substrates having dislocation density less than 105 cm−2 can be obtained by ammonothermal growth. Since the ammonothermal method can produce a true bulk crystal, one can grow one or more thick crystals and slice them to produce GaN wafers. We have been developing bulk crystals of GaN by the ammonothermal method. However, we found it quite challenging to avoid cracking of bulk crystals, especially when the total thickness of the bulk crystal exceeds 1 mm. We believe that the cracking problem in bulk group III nitride is a universal problem for any bulk growth methods including the ammonothermal method. Thus, this invention is intended to obtain crack-free bulk group III nitride crystals using any bulk growth method, such as growth in supercritical ammonia or from a melt of group III metals.